Studies will be carried out with three enzymes that catalyze unique reactions possibly very important in turnover and processing of mRNA in Saccharomyces cerevisiae. The first enzyme is an exoribonuclease that produces 5 feet-mononucleotides by a 5 feet greater than 3 feet mode of hydrolysis. The second is an enzyme which decaps m7G5 feet pppRNA-poly(A) of yeast to yield m7GDP and 5 feet-pRNA-poly(A). The last enzyme is an endoribonuclease which preferentially hydrolyzes yeast RNA-poly(A) of high molecular weight. It is stimulated by small nuclear RNAs U1 and U2 of Novikoff hepatoma cells. All three enzymes have been partially purified from a high salt wash of ribonucleoprotein particles of yeast. The location of these enzymes in essentially the same fractions has led to their combined study in this laboratory. The enzymes will be further purified, localized in the cell, and several features of the reactions will be studied. Factors involved in the decapping reaction and features of RNA molecules that determine the rate of decapping will be examined. The role of small nuclear RNAs in the endoribonuclease reaction and the specificity of the cleavage reaction will be studied with unique RNAs. Ligation of products of the endoribonuclease by the enzyme itself will be assayed. The characterization of the enzymes should provide evidence on their role in RNA turnover and processing.